1. Field of the Invention
The present invention relates to a semiconductor device for receiving reflected light from printed matter which is irradiated by a light source and converting the reflected light into an electric signal, and more specifically relates to a photo-detecting semiconductor device of a contact type having a light receiving portion positionable in contact with printed matter to directly read the same.
2. Prior Art
FIG. 2 shows a conventional photo-detecting semiconductor device comprised of a photo-detecting element 7' and a driving element 9' for selecting the corresponding photo-detecting element 7' and amplifying an electric signal produced in the photo-detecting element 7'. The photo-detecting element 7' and driving element 9' are formed on a common semiconductor substrate, and are covered with an outer passivation film 2' to protect the elements 7' and 9' from the environment. The elements 7' and 9' are covered with an inner passivation film 4' and are connected to each other through a contact hole 5' and a lead 1'.
FIG. 3 shows a sectional view of a conventional photo-detecting element 7' formed in a semiconductor substrate 3' and covered with a passivation film 2'. The light transmittance T of the element 7' for normal incident light traveling through air 8' is obtained according to the following relation ##EQU1## where ##EQU2## n.sub.1 : refractive index of air n : refractive index of passivation film
n.sub.2 : refractive index of semiconductor substrate PA1 d : thickness of passivation film PA1 .lambda. : wavelength of incident light.
The light transmittance T has maximum and minimum values when .delta.=(2m+1).pi. where m is an integer. FIG. 4A shows the light transmittance T as the function of n.d. Namely, the intensity of light irradiated onto the individual photo-detecting element 7' is varied due to the thickness d of the passivation film 2' and therefore the intensity of the electric signal produced in the individual photo-detecting element 7 is not constant. As shown in FIG. 4A, when the thickness of the passivation film 2' is different between two individual photo-detecting elements formed on the common semiconductor substrate 3', the light transmittance T or the intensity of light irradiated onto the two photo-detecting elements is not equal as indicated by hatched portions a and b.
Stated otherwise, monochromatic incident light is multi-reflected between the opposed major faces of the passivation film and the intensity of the monochromatic light received on the individual photo-detecting element is varied due to the difference of the thickness of the passivation film. In the prior art structure, it is very difficult to form a passivation film of constant thickness over the entire surface of the semiconductor substrate.